If you run both these lines in the script editor, you’ll see scripting.cmdlog=false is logged.

Preferences.SetPreferenceValue( "scripting.cmdlog", false );
LogMessage( "scripting.cmdlog="+Preferences.GetPreferenceValue( "scripting.cmdlog" ) );

So you might think that command logging is disabled. But as soon as your script finishes executing that last LogMessage line, Softimage will turn command logging back on.

Try running this script several times in the script editor:

LogMessage( "scripting.cmdlog="+Preferences.GetPreferenceValue( "scripting.cmdlog" ) );
Preferences.SetPreferenceValue( "scripting.cmdlog", false );

Every time, it will log // INFO : scripting.cmdlog=true.

Softimage always resets scripting.cmdlog, because we can’t let any script just leave command logging disabled. Over the years, it’s been the cause of too many support calls and complaints. So, you can disable command logging for the duration of your script, but not permanently.